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Extending LANs Fiber modems Repeaters Bridges Switches Distance limitation in LANs MAC protocols such as CSMA/CD require time proportional to the length of the cable Electrical signal weakens with distance

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extending lans
Extending LANs
  • Fiber modems
  • Repeaters
  • Bridges
  • Switches
distance limitation in lans
Distance limitation in LANs
  • MAC protocols such as CSMA/CD require time proportional to the length of the cable
  • Electrical signal weakens with distance
  • Network designers choose a combination of capacity, delay and distance that can be achieved within a given cost
fiber optic extensions
Fiber optic extensions
  • Fiber modems extend connection between computer and transceiver
  • Join Ethernet cables (segments) together
  • Amplified signal - no knowledge of frames
  • Deals with signal strength, but not delay
multiple repeaters
Multiple repeaters
  • Ethernet standard says no more than four repeaters between two computers
Fiber modems can be used between repeaters for long distance extensions
  • Biggest problem with repeaters is that they transmit all signals including collisions and noise which limits scalability
  • Connect two segments, but work at the frame level
  • Use promiscuous mode and forward all frames
  • Don’t forward erroneous frames (e.g., collisions and noise)
frame filtering
Frame filtering
  • Only forward a frame if necessary
    • destination is on the other segment
    • broadcast address is used
  • Bridge learns which segment a computer is on when that computer sends a frame
  • When a frame arrives the bridge
    • extracts source address and updates knowledge
    • inspects destination address for forwarding
planning a bridged network
Planning a bridged network
  • Propagation principle - a bridge will only forward frames as far as is necessary
  • Bridges allow communication on separate segments to occur at the same time
  • Plan the network so that computers that communicate frequently are on the same segment
  • May be possible to improve an existing LAN’s performance by adding a bridge
a cycle of bridges
A cycle of bridges
  • Compute a distributed spanning tree
  • Switch - a single electronic device that transfers frames between computers
  • Whereas a hub simulates a shared medium, a switch simulates a bridged LAN with one computer per segment
  • Advantage is greater data rate due to parallelism
  • Some organisations combine hubs and switches to reduce cost
  • Fiber optic extensions
  • Repeaters and multiple repeaters
  • Bridges
    • frame filtering
    • bridging distances
  • Switches
summary of lans
Summary of LANs
  • Locality of reference
  • LAN topologies
  • MAC protocols, especially CSMA/CD
  • Hardware addressing and frame types
  • LAN wiring
  • Extending LANs
exam question
Exam question
  • http://www.cs.nott.ac.uk/~mvr/2-CCN.doc